Eli5: How does magma/lava from volcanoes work?

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What actually pushed the magma out of the volcano? Where does the pressure come from? Also, some volcanoes explode and shoot lava high into the air, and some simply ooze lava slowly out of the top. What circumstances cause these differences?

In: Earth Science
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The pressure comes from all the rock sitting over the magma…all that solid rock making up the volcano has weight that’s pushing down on the magma, pressurizing it. Normally it can’t go anywhere, like air in a balloon. But if you put a hole in it (the volcano) the pressurized magma comes out.

The size/shape of the hole, the thickness of the magma, and the pressure all influence how fast it can get out. Some magma has a lot of dissolved gas in it too, which causes it to expand as the pressure relieves. This is where foamy lava (pumice, etc.) comes from.

It’s usually plate tectonics shifting. Massive sheets of rock sliding into one another with enough force and pressure to melt some. It then basically gets squeezed up to the surface through whatever cracks and openings it can find, which are usually volcanoes.

Whether it explodes or just sorta oozes out depends on the magma. There’s a lot of gases that can get trapped in the magma as it melts. If it’s soft and liquidy magma then that’s not a problem, the gases just sorta escape gradually without becoming an issue.

If the magma is thick then it’s hard for the gases to escape and they’ll keep building up until finally there’s just too much gas and they erupt violently. Whether the magma is thick or thin mostly depends on what kind of rock it’s made of.

It’s sorta like shaking a soda bottle. If you open it normally, the gases just fizzle up without an issue. If you shake it though, then a lot of extra gas gets dissolved in the soda and it’ll explode when you open it.